Mozilla last week released the first public preview of a Firefox browser that runs in Windows 8's touch-first "Modern" or "Metro" user interface (UI).

The Firefox app for Modern -- the UI dubbed "Metro" until Microsoft ditched the term over trademark issues -- will be partnered with the traditional desktop browser in a package that may appear in early January 2013 as Firefox 18.

Asa Dotzler, product manager for Firefox, announced the preview last Thursday in a short post to a Mozilla blog.

"Over the coming weeks and months, we'll be adding more features, tightening up Windows integration, improving performance and responsiveness, and finishing up all the necessary work to deliver a first-class Firefox experience for Windows 8," Dotzler wrote.

Users running Windows 8 RTM (release to manufacturing), the final code Microsoft started distributing in mid-August, can download and install a Modern-ized Firefox from Mozilla's "Nightly" channel, a rough-edged, in-development build that precedes the better-known Aurora, Beta and Final stages for each edition.

As Dotzler acknowledged, Firefox for Modern harbors bugs and omits features that should appear in the final. Computerworld, in fact, was unable to run the Firefox app in the Modern UI on a 32-bit version of Windows 8 RTM. The touch-sensitive browser, however, did launch and work properly in Windows 8 64-bit.

Mozilla engineers are investigating the issue.

Brian Bondy, a Firefox platform engineer who has been working on the Modern version for Windows 8 for most of the year, was more specific than Dotzler in describing what is in the preview, and to a lesser extent, what will be in the future. Last spring, Mozilla committed to creating a browser for Windows 8's new UI, the first of Microsoft's rivals to do so; Google followed suit a month later.

Browsers are a special case for Windows 8. After a several-month delay last year, Microsoft allowed hybrid desktop-Modern browsers. Modern-style enabled browsers can run outside the normal security sandbox required for all other apps, and have access to most Windows APIs (application programming interface) on the classic desktop side, as well as the new WinRT API, the backbone of the Modern side of Windows 8 application development.

The category also gets an important pass from Microsoft: A Modern-enabled desktop browser circumvents the Windows Store -- the Microsoft-curated distribution channel -- and when installed on the Windows 8 classic desktop, simultaneously installs the Modern version. The biggest caveat for a Windows 8 hybrid browser is that only the default browser -- which is set by the user -- can run in the Modern UI. During setup Windows 8 assigns Internet Explorer 10 (IE10) as the default browser.

Bondy had made the case earlier that it was important for Mozilla to craft a Modern Firefox app because of the default-browser issue. "Even if a user spends most of their time in the desktop [UI], having a really good [Modern] browser may be enough for the user to change their default," Bondy argued in April. It's unclear when Mozilla will wrap up work on the Modern app. Although the preview is pegged as Firefox 18, it's actually a spin-off of the normal Nightly channel, and so it does not yet have a definitive timetable.

Firefox 18 reaches the Aurora channel -- analogous to an "alpha" build -- next week, and will shift to Beta in late November before launching Jan. 9, 2013. It seems unlikely the Modern app will be ready soon, certainly not in time to be added to the Aurora channel for Firefox 18. Previously, Dotzler said that he didn't believe a Firefox app for Windows 8 would be further along than a "late-stage Beta" this year.

Microsoft will launch Windows 8 on Oct. 25 during a several-hour event, and start selling the upgrade the next day. Also on Oct. 26, PC makers will begin selling systems pre-loaded with Windows 8.

Windows RT, the touch-only spin-off of Windows 8 designed for tablets, is an entirely different story, however, and complicates Mozilla's work. Unlike Windows 8, Windows RT bars all third-party developers from accessing the limited "desktop" mode; only Microsoft's programmers can call the APIs there, which they do to power much of the Modern version of IE10 on the tablet OS.

Mozilla howled, and in May accused Microsoft of anticompetitive behavior by purposefully blocking others from building browsers that could effectively compete with IE10. So although Firefox for the Modern UI may work on Windows RT -- Microsoft has promised that apps written for that mode on Windows 8 will also run on the Modern-only Windows RT -- it will likely require not only considerable recoding, but will, Mozilla has said, be significantly slower than IE10 in rendering JavaScript.

In June, Google launched a preliminary version of Chrome for the Windows 8 Modern environment, but that app is less a reworking of its sibling than a minimalist effort that discards Microsoft's design guidelines. Google has not set a launch date for its Modern-style browser, but looks likely to debut it alongside Chrome 23, which would ship sometime next month.

The Modern-enabled preview of Firefox 18 can be downloaded from a Mozilla server ( FTP download), but caveats abound: Users must be running Windows 8 RTM, and must set the browser as the operating system's default by following several steps outlined by Bondy in his blog.

The next non-Modern edition of Firefox, version 16, will launch Tuesday, Oct. 9.